There is no recent study on the prevalence of overweight and obesity in patients with type 1 diabetes mellitus (T1DM) in Japan. Being overweight has a significant effect on the metabolic condition and glycemic control of such patients. In the present cross-sectional study, we investigated the effects of body mass index (BMI) on lipid profile, blood pressure, and glycemic control in patients with T1DM. In total, 1486 patients with T1DM (including 401 patients with early onset T1DM who were <20 years of age at diagnosis) were included. Patients were divided into four groups according to their BMI, and glycosylated hemoglobin (HbA1c), daily insulin dose per kg body weight, lipid profile, and blood pressure were compared between groups. We found that 15.7% of all patients were overweight (BMI ≥ 25.0 kg/m²) and 2.0% were obese (BMI ≥ 30.0 kg/m²), compared with 17.5% and 2.0%, respectively, in the early onset T1DM subgroup. Significant changes in lipid profiles and blood pressure were found with increasing BMI in both the entire population and the early onset T1DM subgroup. In the entire study population HbA1c and the body weight-adjusted daily insulin dose were significantly higher in patients with a BMI ≥ 23 kg/m² compared with those with a BMI<23 kg/m²; however, this was not the case in the early onset T1DM subgroup. This difference may be due to the relatively small number of patients in that subgroup. In conclusion, the prevalence of overweight and obesity in patients with T1DM was less than that in the normal Japanese population. For patients with T1DM, being overweight was associated with higher blood pressure and dyslipidemia. Furthermore, we cannot exclude an association between being overweight and the need for higher daily doses of insulin.